----------------------------Original message---------------------------- This message is in response to the questions and comments of Sue Haffner and others regarding the digitization of aerial photography. After a great deal of experimentation we at the Map & Geography Library at the Univ. of Illinois are about to begin an expanded pilot project involving the digitization of our pre-1950 aerial photography. Our goal in digitization was (and is) to get the best reproduction possible for a permanent archive. I collaborated with a remote sensing analyst from the Illinois Natural History Survey and a photo interp. person from the Illinois State Geological Survey in the experimentation. A detailed discussion of this experimentation is too long for an email discussion list. But I will try to give a concise list of our findings. 1. We are extremely happy with the results at this point. Our old prints, in general, support a scan rate of 42 microns per pixel giving us a ground resolution of 3.8 feet per pixel. This compares to 3.28 ft (1 meter) for the DOQs. 2. We have developed a consistent scheme to determine optimal scan rates which can vary for different flight dates and depend upon quality of the print, the paper, and other factors. Negatives or transparencies can support, in general, higher scan rates. 3. The equipment of choice is a Crossfield drum scanner. The Crossfield starts at $250,000 and requires a great deal of maintenance. Thus, we decided that we would not buy one. 4. The primary companies that are experienced with aerial photography are those that contract with the USGS for DOQ production. We solicited samples from three companies, two very large companies and a young local company with a Crossfield. None of these were able to satisfy us with the quality we were looking for as a first effort. However, the local company, (Scantech Color Systems) worked very closely with us to satisfy us, investing many hours and dollars in the experimentation, and is prepared to handle aerial photo digitization requests outside the state as well. 5. Scantech has given us an estimate of $34 per print for the pilot project, (a cadillac scan for an escort price), and expects the price to go down after the pilot. 4. The pilot project will consist of a single county. DOQ production for that county will be completed in December. Using the Digital Terrain Model resulting from DOQ production and the DOQ, we expect to have the old photos rectified, and are soliciting samples and prices from all the companies that contract with the USGS for DOQ production and two Canadian companies as well. 5. We (our remote sensing analyst) registered two photos to their corresponding DOQ as part of the experimentation. I'm not sure it is possible to overstate the effect of seeing the 1939 and 1988 photos overlayed. The two images were compared mathematically so that change and intensity of change could be displayed in different colors on the screen. 6. We presented the results of our experimentation with numerous slides a la digital camera at the "GIS in Illinois" conference early this week with very positive comments. At this point we have four other counties who are interested in funding the scans for their counties, and various other large scale funding arrangements are being discussed. Sorry this is so long; if anyone wants more information or a more detailed report, please let me know. Li Hunt Map & Geography Library Univ. of Illinois [log in to unmask]